From 1956 to 1957, he was the Parliamentary Assistant to the Secretary of State for External Affairs. From 1963 to 1965, he was the Associate Minister of National Defence. In 1965, he was the Minister of Public Works. From 1965 to 1967, he was the Minister of Justice and Attorney General of Canada.

Cardin was the first Canadian politician to bring the public's attention to the Gerda Munsinger affair. During taunts by Conservative MPs in the House of Commons in March 1966, Cardin shouted out across the floor of the House, "What about Monsignor?" Although he got the name wrong, and later insisted that he thought Munsinger had died, the media brought attention to the issue, and there was a federal inquiry that caught the public's attention due to its implications to national security during the Cold War.